Underwriters Laboratories requirements for arc fault circuit interrupters (AFCI) require the AFCI to respond to certain arcing conditions on a branch circuit, i.e., a portion of an AC electrical distribution system, that the AFCI is intended to protect. When the AFCI detects an arc signature indicative of an arc fault, the AFCI interrupts the flow of electrical current on the protected branch circuit before the arcing condition causes flammable ignition of nearby combustibles. Arcing conditions may also occur elsewhere in the electrical distribution system, such as in the power utility or on other branch circuits, which the AFCI is not able to interrupt due to the location of the AFCI. Such arcing conditions produce the same arc fault signatures as those on the protected branch circuit and are sensed by the AFCI.
In order to meet Underwriters Laboratories requirements, the AFCI must not trip due to upstream arc mimicking noise or arcs associated with power utility arcing or alternate branch circuit arcing, but must respond to similar signals that are on the protected circuit. There is a need for an AFCI that discerns the location of the arc fault signals and responds only to those produced in the protected branch circuit.
“Downstream” refers to the branch circuit in which the AFCI is incorporated and that the AFCI is intended to protect. “Upstream” refers to the remainder of the electrical distribution system that the AFCI is unable to protect, for which tripping is considered nuisance tripping. Thus, upstream and downstream are always determined with reference to the location of the AFCI.